1. Field of the Invention
The present invention relates generally to apparatus and methods for accomplishing separation of liquids of different densities in fluid streams from underground wells. In one aspect, the invention also relates to control of the oil-water interface in production reservoirs as well as the prevention of the problems associated with coning and reverse coning.
2. Background of the Related Art
In most hydrocarbon production areas, a relatively permeous layer or zone containing hydrocarbons is trapped horizontally between layers of relatively impermeable rock. There exists a natural separation of gas, oil and water within the zone. The gas, being the lightest of the three, tends to migrate toward the top of the production zone. The water tends to migrate toward the bottom of the production zone leaving an oil layer sandwiched in the middle. The interface between the gas and oil is often referred to as the gas-oil contact, while the interface between oil and water is often referred to as the oil-water contact. During an oil production operation, the object is to remove as much oil from the formation without removing the water below it. It may or may not be desired to remove the gas. In order to prevent removing water with the oil, however, production perforations into a hydrocarbon production zone are normally made above the oil-water contact. Oil is drawn into the wellbore through these production perforations and then transmitted to the surface through production tubing.
Because water has a higher relative permeability than oil, a phenomenon known as coning tends to occur wherein the water is drawn upward through the reservoir toward the production perforations as the oil is drawn off. If the water succeeds in reaching the production perforations, it may block or substantially reduce further entry of oil into the wellbore, thereby leaving pockets of oil behind which cannot be recovered. Additionally, the presence of water in the wellbore and production tubing is undesirable as it increases the hydrostatic head within the wellbore.
Past efforts at preventing coning have focused on locating the production perforations to penetrate the oil layer as high as possible above the oil-water contact in an effort to reduce or delay water coning. Although this approach will be effective until the oil layer is reduced, it has the disadvantage that the perforated interval, or interval between the top of the production perforations and the bottom of the production perforations, cannot cover the full span of the oil leg that remains in the reservoir.
An alternative approach to preventing coning has recently been proposed in which a well is completed so that there are separate perforations for production fluid and produced water from the reservoir. The proposal was outlined by B. R. Peachey and C. M. Matthews in "Downhole Oil/Water Separator Development," in Vol. 33, No. 7, The Journal of Canadian Petroleum Technology (September 1994) at 17-21. In the proposal, the production tubing is packed off against the annulus of the wellbore by a packer which is set approximately at the level of the oil-water contact. The production perforations would be located above the packer so as to penetrate the oil layer and permit oil to enter the wellbore above the packer. Produced water perforations would then be located below the packer so as to penetrate the water layer so that water will enter the wellbore below the packer. The proposal envisions incorporating a dual stream pump arrangement into the production tubing string which includes a low volume, high head oil pump and a high volume, low head water pump. The water would be pumped either to a lower zone in the same reservoir or to a separate zone suitable for water disposal that is accessible from the same well. The oil pump would pump separated oil through the production tubing toward the surface for recovery.
The use of offsetting produced water perforations creates a pressure sink which aids in reducing coning by drawing off water at a location below production perforations and will even generate some "reverse coning" of the fluids in the near wellbore area. Reverse coning occurs when oil from the oil layer migrates downward through the formation toward the water perforations. Unfortunately, reverse coning may ultimately result in loss of production fluid through the produced water perforations located below the packer. This is undesirable. The present invention provides a solution to the problems found in the prior art.
In another aspect of the invention, intelligent and semi-intelligent production systems are described which are capable of monitoring the approximate position of the oil-water contact in the surrounding formation and adjusting pump and flow rates to adjust the position.